1. Field of the Invention
The present invention generally relates to system for testing automatic testing machines, and more particularly to method and apparatus for testing the accuracy of signal voltage levels on a digital testing device.
2. Discussion of the Related Art
A variety of automatic test equipment (ATE) have long been known for testing electronic circuits, devices, and other semiconductor and electronic products. Generally, automatic test equipment are divided into two broad categories, analog testers and digital testers. As the names imply, analog testers are generally designed for testing analog circuit devices, while digital testers are designed for testing digital circuit devices. Digital testers, as is known, generally include a testing device having a number of internal circuit cards or channels that generate programmably controlled test signals for testing and evaluating a Device Under Test (DUT). More specifically, ATE are programmably controlled to be adapted or configured to testing a variety of devices in a variety of ways. This is achieved by programming output signals to inject a certain signal (or signal transition) to a certain pin or signal line on a DUT. In this regard, a digital tester generally includes a test head whereby electrical signals are input to and output from the tester. The test head comprises a number of connectors, each defining a channel, which may be connected via cable or otherwise to a device under test. The electronics within the digital tester may then input and output signals to/from a DUT via the test head.
By way of an extremely simple illustration, consider a digital tester that is configured to test a wafer containing, among other things, a two input AND gate. The digital tester may be configured to apply a logic one on the two signal lines that correspond to the inputs of the AND gate, then receive the signal on the signal line corresponding to the output to ensure that it is driven to a logic one. The tester may then be configured to alternatively apply logic zero signals on each of the two signal lines corresponding to the AND gate inputs, in order to verify that the output of the AND gate transitions from a logic one to a logic zero in response. Although such a test will verify the functional operation of the AND gate, additional tests must be executed to verify timing and other aspects of the AND gate.
For example, assume that the two input signals to the AND gate are a logic one and a logic zero, the output is also a logic zero. When, however, the second input transitions from a logic zero to a logic one (whereby both inputs are at a logic one) then the output of the AND gate transitions from a logic zero to a logic one. It is important, however, that the output fully transition to a high state. More specifically, and as is known, digital logic devices operate, generally, in a range of zero to five volts, where zero volts is a logic zero and five volts is a logic one. As devices become loaded, however, they often fail to fully drive an output signal to five volts. Accordingly, a range is presumed to be a logic one. For example, any output above 2.4 volts (depending upon device specifications) may be treated as a logic one. Thus, continuing with the AND gate example, when both inputs are above 2.4 volts, the output level should also drive to at least 2.4 volts. Suppose an automatic tester incorrectly interpreted a 2.3 volt output voltage as 2.4 volts. The tested component may be deemed as good, when in fact it was out of tolerance with the manufacturer's specifications. It will be appreciated that the example given above is an extremely simple example and is presented merely for purposes of illustration, and, as will be appreciated by those skilled in the art, digital testers are much more sophisticated and are capable of performing much more sophisticated and complex testing routines.
Accordingly, an extremely important aspect of ATE, including digital testers, is that the testing equipment maintain extremely accurate tolerances. Otherwise, it will not be clear whether measured values of a DUT reflect errors or discrepancies within the DUT, or whether errors or discrepancies result from component or other tolerance variations in the ATE components. In this regard, automatic test equipment manufacturers generally provide a set of built-in-test (BIT) routines for the automatic test equipment. However, and as will be further described below, these manufacturer provided BIT routines have proven to be generally inadequate in today's market of high-speed components and high volume manufacturing facilities.
A digital tester generally comprises a number of similar (if not identical) channels, each comprising a number of drivers and receivers for driving output signals or receiving input signals (output from the tester or input to the tester). These channels (which are generally comprised on a single circuit board) often include a Parametric Measuring Unit (PMU) or some other type of measuring device for sensing and measuring the magnitude of a signal. As is known, PMUs are known for calibrating on-board components. Thus, when a given driver is programmed to generate a signal of a certain amplitude, the on-board PMU may monitor and measure that signal in order to determine whether the amplitude of that signal is the proper value.
This type of further testing may be accomplished by using accurate, external testing equipment in a manual fashion. For example, voltmeters may be used for testing voltage levels, and oscilloscopes may be used for evaluating timing aspects. However, it will be appreciated that external testing of an ATE is extremely time consuming, and therefore adversely impacts production quantities. A particular problem that has been noted in recent years specifically relates to a digital tester's ability to self-test for accuracy. As a result of the general failure of testing devices in this respect, circuit designers have been known to design tests for specific testers. That is, a circuit designer may design a test having test parameters such that the test will pass the testing requirements of a given automatic testing device, while failing those same tests on other testers. Charts have been generated that keep track of certain characteristics of various automatic testing equipment. Circuit designers have used the information contained within these charts in order to design test parameters to pass the test of a given tester. Clearly, however, this approach adversely affects the overall yield of a designed product, as well as material flow through the high volume manufacturing environment, and is disfavored.
Accordingly, there is a need to develop a system for improving the accuracy of automatic testing equipment, specifically digital testers, by verifying with a high degree of accuracy the performance of the digital testers.